There is an almost endless number of physical and chemical processes employed today wherein proper and efficient operation depends upon accurate measurement, and often implementation of controls or corrective measures are taken based on these measurements. Sewer flow is particularly a critical such measurement.
Sewer flow basically involves the determination of liquid height in a sewage flow containment and average velocity of flow (different levels may flow at different rates) in that containment. These quantities may be separately measured or one measured, particularly liquid height, and the other computed with known equations, known as Manning's equations. Liquid height measurements are typically measured via an immersed sensor or an ultrasonic distance measurement made from a reference position down to the liquid level of sewage, the latter being feature of this invention. By such techniques, liquid level can generally be quite accurately determined. However, the other quantity, average velocity of the volume of flow, has not been readily susceptible of reliable measurement, particularly over a significant period of time.
Perhaps the oldest approach, which dates back a number of years, has been to employ a sonic transducer either within the fluid or on the outside of the containment to transmit a sonic signal through the sewage and to detect signals (typically echoes from moving objects or particles in the sewage) and measuring the change in frequency which has occurred due to the velocity, or Doppler, effect on the signal. Such a system is described in U.S. Pat. No. 4,397,191. Another approach, described in U.S. Pat. No. 4,083,246, employs an electromagnetic sensor Which senses the velocity of flow immediately around the sensor.
In the Doppler case, the problem has been to somehow select from many signals the one representing the average flow for the sewage since the immediate signal information does not reliably indicate this factor. In practice, the Doppler signal is chosen by counting the number of signal zero crossings or by a phase lock loop which locks onto one signal as a chosen average velocity signal. In the case of the former, substantial velocity errors (from average velocity) arise from the fact that one has typically a number of signals which contribute to zero crossings, and it is almost happenstance that together they represent a true average, or average velocity. In the case of a phase lock loop, such a system tends to lock onto the loudest signal which, again, may or may not be representative of average velocity. In any event, signals errors will be present almost as many times as accurate signals.
In the case of a magnetic-type device, typically it must be positioned at one level in the containment and thus only it determines velocity at that level, which may or may not be representative of average flow. To correct for this, the measured velocity output signal is compensated by a factor which is a function of depth of fluid. The basic problem, however, is that with any significant sludge around the sensor, a constant hazard in the case of sewer pipes, velocity accuracy drops off quite significantly with any extended usage.
As indicated in U.S. patent application Ser. No. 440,502, and with respect to which the applicant is a co-inventor, there has been determined a new approach to Doppler signal analysis enabling extremely accurate determination of average velocity. It is accomplished by scanning or otherwise detecting the range of Doppler velocity signals present, as by a Fourier transform, and to then select as the frequency indicative of average velocity the one which is on the order of 90% of the highest frequency present. By the employment of this approach, there is enabled in the present system a particularly accurate determination of fluid flow and, as a consequence, a most accurate determination of inflow and infiltration measurements, as generally discussed in U.S. Pat. No. 4,630,474.